1. Field of the Invention
The present invention relates to a fuel injection system for supplying fuel into an engine by injection. Specifically, the present invention relates to a fuel injection system, which draws fuel from a fuel tank with the use of an electric actuator such as an electric motor.
2. Description of Related Art
A fuel injection system for supplying fuel to an engine by injection includes a high-pressure pump, which pressurizes the fuel to a high pressure and supplies the fuel to the engine through injection valves (injectors), and a low-pressure pump, which draws the fuel from a fuel tank and supplies the fuel to the high-pressure pump. The high-pressure pump has a rotary shaft rotated by the engine. If the rotary shaft rotates, the high-pressure pump suctions and pressurizes the fuel, which is supplied by the low-pressure pump, to the high pressure, and pressure-feeds the fuel to the injectors. The low-pressure pump is attached to an end of the rotary shaft of the high-pressure pump. If the rotary shaft rotates, the low-pressure pump draws the fuel from the fuel tank and supplies the fuel to the high-pressure pump. Thus, the high-pressure pump and the low-pressure pump are driven by the engine and supply the fuel to the engine according to an engine rotation speed, or a fuel quantity required by the engine.
In recent years, a fuel injection system employing electrically-driven high-pressure pump and low-pressure pump driven by an electric actuator (for instance, an electric motor) has been proposed, for instance, as disclosed in JP-A-H09-209870 or JP-A-2000-179427, instead of the engine-driven high-pressure pump and low-pressure pump driven by the engine. The electrically-driven fuel injection system, specifically, the fuel injection system equipped with the electrically-driven low-pressure pump (the electric low-pressure pump), has advantages over the fuel injection system equipped with the engine-driven low-pressure pump as explained below.
First, the engine-driven low-pressure pump cannot supply a larger quantity of the fuel than a quantity corresponding to the engine rotation speed. Therefore, there is a possibility that the fuel supply becomes deficient in a low-rotation speed period occurring immediately after a start of the engine, for instance. In contrast, the electric low-pressure pump can supply a constant quantity of the fuel irrespective of the engine rotation speed. Therefore, the deficiency in the fuel supply does not occur in the low-rotation speed period occurring immediately after the start of the engine.
Secondly, since the engine-driven low-pressure pump is attached to the end of the rotary shaft of the high-pressure pump, the system requires another pump for filling a fuel passage leading from the fuel tank to the low-pressure pump with the fuel when the engine is restarted after an engine stall or when the engine is shipped. In contrast, the electric low-pressure pump can be mounted near the fuel tank because of a large freedom degree of a mounting position of the electric low-pressure pump. Therefore, the fuel pump for filling the fuel passage is unnecessary.
Next, an example of a fuel injection system 101 of a related art having an electric low-pressure pump 100 will be explained based on FIG. 6. The fuel injection system 101 includes a high-pressure pump 102, the low-pressure pump 100, and a common rail 105. The high-pressure pump 102 pressurizes the fuel to a high pressure and supplies the fuel to an engine. The low-pressure pump 100 is driven by an electric motor 103 as an electric actuator. Thus, the low-pressure pump 100 draws the fuel from a fuel tank 104 and supplies the fuel to the high-pressure pump 102. The common rail 105 accumulates the fuel, which is supplied by the high-pressure pump 102, at an injection pressure, at which the fuel is injected into the engine.
The high-pressure pump 102 supplies the high-pressure fuel in the common rail 105 into the engine through the common rail 105 and injection valves (injectors) 108 by injection. The high-pressure pump 102 is formed with a cam mechanism 111 driven by the engine and with pressurizing chambers 112 capable of expanding and contracting. The high-pressure pump 102 has multiple pressurizing portions 113 and a suction control valve (SCV) 114. The pressurizing portions 113 are driven by the cam mechanism 111. Thus, the pressurizing portions 113 introduce the fuel into the pressurizing chambers 112 and pressure-feed the suctioned fuel to the injectors 108. The SCV 114 regulates a suctioning quantity of the fuel suctioned into the pressurizing chambers 112 out of the fuel supplied from the low-pressure pump 100.
The cam mechanism 111 includes a rotary shaft 117, a cam 118 in the shape of a circular column, and a cam ring 119. The rotary shaft 117 is rotated by the engine. The cam 118 is eccentrically fitted to the rotary shaft 117. The cam ring 119 slidably accommodates the cam 118.
Each pressurizing portion 113 includes a plunger 123 and a spring 124. The plunger 123 is slidably accommodated in a cylinder 122 and driven by the cam mechanism 111 away from the rotary shaft 117. The spring 124 biases the plunger 123 toward the rotary shaft 117. A plunger tappet 125 is disposed on a tip end of the plunger 123 on the rotary shaft 117 side. A biasing force of the spring 124 brings the plunger tappet 125 into sliding contact with a siding surface formed on an outer periphery of the cam ring 119. The pressurizing chamber 112 is provided by an inner peripheral surface of the cylinder 122, an end surface of the plunger 123 opposite from the rotary shaft 117, and the like. The multiple pressurizing portions 113 are formed around the rotary shaft 117 at an equal angular interval (for instance, an interval of 180° or 120°).
A valve member of the SCV 114 is driven by a magnetic force generated by energization of a solenoid of the SCV 114. A value of current for the energization is controlled by duty cycle control in order to regulate a valve opening degree. If the energization of the solenoid of the SCV 114 is stopped, the valve opening degree of the SCV 114 is changed to a fully opened state or a fully closed state by a biasing force of a spring and the like.
The cam 118, the cam ring 119 and the plunger tappet 125 are accommodated in a cam chamber 128. The cam chamber 128 is supplied with part of the fuel, which is supplied from the low-pressure pump 100 and is circulated, as lubricating fuel. Thus, seizing due to the sliding contact between the cam 118 and the cam ring 119 and seizing due to the sliding contact between the plunger tappet 125 and the cam ring 119 can be prevented. A restrictor 129 limits the supply of the lubricating fuel.
If the rotary shaft 117 is rotated by the engine, the cam 118 revolves around a central axis of the rotary shaft 117. Accordingly, the plunger 123 reciprocates once in the cylinder 122 while the rotary shaft 117 makes one revolution. More specifically, if the rotary shaft 117 makes one revolution, the plunger 123 moves from a position where the volume of the pressurizing chamber 112 is maximized to another position where the volume is minimized, and then, the plunger 123 returns to the position where the volume is maximized. Meanwhile, the plunger tappet 125 slides on the sliding surface of the cam ring 119.
If the volume of the pressurizing chamber 112 is maximized, suctioning operation for suctioning the fuel into the pressurizing chamber 112 ends and fuel pressure-feeding operation for pressure-feeding the fuel from the pressurizing chamber 112 starts. Then, the fuel pressure in the pressurizing chamber 112 remains high while the volume of the pressurizing chamber 112 changes from the maximum volume to the minimum volume. Thus, the high-pressure fuel is pressure-fed from the pressurizing chamber 112 to the common rail 105. If the volume of the pressurizing chamber 112 is minimized, the pressure-feeding operation for pressure-feeding the high-pressure fuel from the pressurizing chamber 112 ends and the fuel suctioning operation for suctioning the fuel into the pressurizing chamber 112 starts. The fuel pressure in the pressurizing chamber 112 remains low while the volume of the pressurizing chamber 112 changes from the minimum volume to the maximum volume. Thus, the fuel is suctioned into the pressurizing chamber 112.
The low-pressure pump 100 is a pump having a publicly known structure, which draws the fuel from a fuel tank 104 and supplies the fuel to the high-pressure pump 102 by rotating an impeller 130 thereof. The impeller 130 of the low-pressure pump 100 is rotated by an electric motor 103 to draw the fuel from the fuel tank 104 and to supply the fuel to the high-pressure pump 102 mainly through the SCV 114. An excess quantity of the fuel out of the fuel supply quantity of the low-pressure pump 100 is released to the fuel tank 104 by a pressure regulation valve 131.
The supply quantity of the low-pressure pump 100, or a supply pressure of the low-pressure pump 100, is constant regardless of a change in the pressure-feeding quantity of the high-pressure pump 102. Therefore, the pressure regulation valve 131 regulates the pressure at an inlet of the SCV 114. More specifically, if the pressure-feeding quantity of the high-pressure pump 102 decreases, an opening degree of the pressure regulation valve 131 increases and a quantity of the fuel released to the fuel tank 104 increases. If the pressure-feeding quantity of the high-pressure pump 102 increases, the opening degree of the pressure regulation valve 131 decreases and the quantity of the fuel released to the fuel tank 104 decreases.
The fuel injection system 101 is controlled by controlling means 135.
For instance, the controlling means 135 controls an injection quantity or injection timing of the fuel injected from the injectors 108 into the respective cylinders in accordance with sensing signals of engine rotation speed sensing means 136 and accelerator position sensing means 137, which sense the requirements of the engine.
The controlling means 135 regulates the suctioning quantity of the SCV 114 so that the fuel pressure in the common rail 105 (a common rail pressure) substantially coincides with the injection pressure of the injectors 108 in accordance with a sensing signal outputted from common rail pressure sensing means 138, which senses the common rail pressure. The suctioning quantity of the SCV 114 is regulated by duty cycle control of the current value supplied to the solenoid of the SCV 114.
The electric low-pressure pump 100 of the related art needs to continuously supply the fuel corresponding to the maximum value of the fuel quantity required by the engine lest the fuel supply quantity become less than the fuel quantity required by the engine. Therefore, the fuel supply of the electric low-pressure pump 100 of the related art is wasteful and power consumption of the electric actuator is large. Therefore, a large amount of the power is required to start the engine (specifically, to energize a starter and the electric actuator 103 of the low-pressure pump 100). As a result, there is a possibility that a size of an alternator needs to be enlarged.
If the fuel injection quantity of the injectors 108 decreases, or if the fuel quantity required by the engine decreases, the valve opening degree of the SCV 114 is decreased in order to reduce the quantity of the fuel pressure-fed to the common rail 105. However, if the valve member of the SCV 114 becomes inoperative at a large valve opening degree due to clogging of extraneous matters or if the fuel leaks from a clearance between the valve member and a valve body, a larger amount of the fuel than the injection quantity of the injectors 108 is pressure-fed to the common rail 105. If an abnormally high common rail pressure occurs or if reduction of the common rail pressure delays as a result, there is a possibility that combustion noise increases.
Moreover, if the electric low-pressure pump 100 is used, there is a possibility that the fuel supply quantity of the low-pressure pump 100 changes with time due to degradation of the electric motor 103 and the like. If the fuel supply quantity of the low-pressure pump 100 changes with time, the suctioning quantity of the high-pressure pump 102, or the quantity of the fuel pressure-fed to the injectors 108, will vary even though the rotation speed of the rotary shaft 117 of the high-pressure pump 102 and the valve opening degree of the SCV 114 are the same as those provided before the change with time. As a result, there is a possibility that a difference between the fuel supply quantity supplied to the engine and the fuel quantity required by the engine increases, and exhaust gas characteristics and the like are deteriorated.